1. Field of the Invention
The present invention relates to a dosing device for adding small amounts of an additive fluid (i.e., a gas or a liquid) to a breathing gas, intended to be supplied to a subject via an inspiratory line in an anaesthetic machine or ventilator during inspiratory phases.
2. Description of the Prior Art
As used herein, the phrase "additive fluid" includes any mixture of gases, diluted or undiluted, as well as pure gases. For instance, NO diluted in N.sub.2 can be used as additive gas, pure xenon can used as additive gas. Liquid anaesthetic also can be used as an additive fluid. Vaporized anaesthetic as a (pure or diluted gas) can be used as an additive fluid.
In inhalation anaesthesia, a small amount of anaesthetic is added to breathing gas for inhalation by a subject. The anaesthetic can be supplied to the breathing gas in the form of gas or liquid. When supplied in form of gas, some of the breathing gas is allowed to bubble through or pass across the surface of a liquid anaesthetic in order to vaporize it. The resulting mixture, containing a relatively high concentration of anaesthetic, is then added to the rest of the breathing gas in a controlled fashion. The anaesthetic can also be delivered directly into the breathing gas as a liquid, whereupon it quickly vaporizes. In the latter case, some kind of heating may be necessary in order to avoid cold gas from reaching the subject and to enhance the vaporization of the liquid.
Other substances that can be supplied, preferably near the subject, are NO, surfactant and other therapeutic substances.
Swedish Patent 448 347 describes a method for mixing gases in predetermined proportions and dosing the gas mixture in conjunction with anaesthesia and respirator care. Different gases are pulse-injected into a mixing chamber for mixing in same and subsequent dosing of the gas mixture. The proportions of the different gases are governed by the volumes of the gas pulses.
Accurate dosing of small amounts of gas or liquid in a safe manner is difficult and problematic. Known dosing devices normally operate with relatively large volumes of the additive, from which dosing volume the additive gas/liquid is taken. If a fault occurs in the device, relatively high doses can be taken from the dosing volume. Even if other safety measures for the patient intercede, the patient may still be exposed to quantities that could cause discomfort, and in the worst case even be harmful or lethal.